chore: upgrade to syn 2

[nmattia]

This upgrades the codebase to syn 2, v1 is now a couple of years old.

[github-actions]

canbench 🏋 (dir: ./examples/fibonacci) 82ae752 2026-08-04 15:44:15 UTC

✅ ./examples/fibonacci/canbench_results.yml is up to date
📦 canbench_results_fibonacci-example.csv available in artifacts

---------------------------------------------------

Summary:
  instructions:
    status:   No significant changes 👍
    counts:   [total 5 | regressed 0 | improved 0 | new 0 | unchanged 5]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  heap_increase:
    status:   No significant changes 👍
    counts:   [total 5 | regressed 0 | improved 0 | new 0 | unchanged 5]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  stable_memory_increase:
    status:   No significant changes 👍
    counts:   [total 5 | regressed 0 | improved 0 | new 0 | unchanged 5]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

---------------------------------------------------
CSV results saved to canbench_results.csv

[github-actions]

canbench 🏋 (dir: ./examples/btreemap_vs_hashmap) 82ae752 2026-08-04 15:44:24 UTC

✅ ./examples/btreemap_vs_hashmap/canbench_results.yml is up to date
📦 canbench_results_btreemap-vs-hashmap-example.csv available in artifacts

---------------------------------------------------

Summary:
  instructions:
    status:   No significant changes 👍
    counts:   [total 3 | regressed 0 | improved 0 | new 0 | unchanged 3]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  heap_increase:
    status:   No significant changes 👍
    counts:   [total 3 | regressed 0 | improved 0 | new 0 | unchanged 3]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  stable_memory_increase:
    status:   No significant changes 👍
    counts:   [total 3 | regressed 0 | improved 0 | new 0 | unchanged 3]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

---------------------------------------------------
CSV results saved to canbench_results.csv